World number one Novak Djokovic encouraged by his serve

Novak Djokovic was happy with his serving after he progressed to the quarter-finals of the Australian Open.

The most pleasing aspect of Monday's straight-sets win over Gilles Muller was the standard of Novak Djokovic's serve, claimed the world number one.

Four-time Australian Open champion Djokovic did not drop serve en route to a 6-4 7-5 7-5 win over Muller on Rod Laver Arena.

Muller was by no means out of his depth against Djokovic, who set up the victory with a break in each of the three sets at Melbourne Park.

But Djokovic was simply too good on serve as he sent down 13 aces, winning 82 per cent of his first serve points in the process, something that clearly pleased the Serbian star.

"I think I have to take the positives from and definitely what stands out is the serve," Djokovic said.

"I got a lot of aces, got a lot of free points there.

"That's something that obviously encourages me for the rest of the tournament."

Djokovic, who has not lost a set ahead of his quarter-final showdown with eighth seed Milos Raonic, added: "I got the crucial three breaks at the important moments, especially in the second and third at five-all, [and] managed to play some good passing shots.

"I was trying just to make him play an extra shot.

"He takes away the time from the opponent. His serves and volleys, very difficult serve to read.

"I have to motivate myself and obviously use the off days to work on a few things. There are a few things I think I can do better. Overall it's a positive feeling going into the quarters."